1. Technical Field
The present invention relates to a glass base material elongation method in which the glass base material is heated and elongated into a glass rod with a smaller diameter.
2. Related Art
When manufacturing a quartz glass rod, which can be exemplified by an optical fiber preform, after manufacturing a glass base material in advance with a large diameter, an elongation apparatus including a heating furnace is used to elongate the glass base material and obtain a glass rod with a smaller diameter. The glass rod resulting from the elongation in the elongation apparatus has a relatively large outer diameter fluctuation of ±3%, for example. Therefore, the glass rod is again elongated with greater precision in an elongation apparatus using a burner, referred to as a glass lathe, as the heating source, thereby adjusting the glass rod to have outer diameter fluctuation of ±1% or less, which is desired for the product.
In recent years, when manufacturing optical fiber from an optical fiber preform, optical fiber manufactured from a larger optical fiber preform has the advantage of a greater capacity utilization rate. Therefore, an optical fiber preform is desired with an outer diameter greater than the conventional diameter of 80 mm, e.g. an optical fiber preform with a diameter of 120 mm or more. However, for a large preform with an outer diameter of 120 mm, the adjustment of the outer diameter with a glass lathe is difficult. This is because, since the heating by the burner is performed in the open air, cooling through radiation occurs at the same time as the heating, and the cooling effect by radiation increases along with the outer diameter, such that a sufficiently high temperature cannot be reached.
When processing with the glass lathe, proposals have been made to use a plurality of burners or cover the region around the heater with heat resistant material. However, using a plurality of burners greatly increases the amount of gas used, which is unfavorable when considering cost. Furthermore, with the method of covering the region around the heater with heat resistant material, foreign material peels away from the surface of the heat resistant material and attaches to the glass rod, which makes it difficult to maintain the desired surface cleanliness. The outer diameter fluctuation in a glass rod obtained from an elongation apparatus is desired to be restricted to within ±1%, which is desired for the product, or to a fluctuation near ±1%.
The following uses FIG. 1 to describe an example of an elongation apparatus used for conventional elongation. The elongation apparatus is formed by three major portions, which are the heating furnace, the feeding section, and the pulling section. The heating furnace includes a heater 1, a water-cooled chamber 3 housing a thermal insulator 2, a top chamber 4 connected to the top of the water-cooled chamber 3, and a lower gas seal 8 attached to the bottom of the water-cooled chamber 3. The feeding portion is formed by a feeding mechanism 7 capable of moving up and down and provided above the heating furnace, a hanging shaft 5 connected to the feeding mechanism 7, and a connecting jig 6. The hanging shaft 5 is inserted into the top chamber 4.
The pulling section is formed by a set of guide rollers 9 below the furnace that is capable of gripping and releasing, a set of upper pulling rollers 10, and a set of lower pulling rollers 11. The guide rollers 9 is formed by heat resistant rollers of carbon or the like, and serves to guide the pulling dummy 14 or glass rod to the center of the apparatus. The upper pulling rollers 10 and the lower pulling rollers 11 are driven by motors, and operate to suitably elongate the glass base material 12 by pulling down the pulling dummy 14 or glass rod gripped by the pulling rollers.
The top end of the hanging dummy 13 provided above the glass base material 12 connects mechanically to the connecting jig 6. In this way, the glass base material 12 is in communication with the feeding mechanism 7 through the hanging shaft 5. The pulling dummy 14 is connected to the bottom end of the glass base material 12. In the case of a glass base material 12 manufactured from a porous glass base material, there is a non-transparent portion 15 in a tapered portion at the top thereof. During elongation, while the glass base material 12 is being pulled down by the feeding mechanism 7, the pulling dummy 14 is pulled downward at a higher speed by the upper pulling rollers 10 and the lower pulling rollers 11, thereby obtaining a glass rod with a smaller diameter from the glass base material 12.
In recent years, in order to decrease the outer diameter variation of a glass rod formed by an elongation apparatus, a variety of techniques have been proposed. For example, Japanese Patent Application Publication No. 2012-076990 describes measuring the outer diameter before elongation, estimating the change in the shape within the heating furnace based on the change in the outer diameter, and accurately controlling the downward pulling speed to form a glass rod having restricted diameter fluctuation over the entire length thereof. Furthermore, Japanese Patent Application Publication No. 2011-116592 describes using a quartz glass rod in which the top chamber is a material with high transparency, enabling visible light and infrared light that propagates upward within the glass base material to efficiently escape to the outside, and preventing heat from being trapped in the tapered portion at the top of the glass base material to restrict the diameter fluctuation near the final portion to be elongated. Yet further, Japanese Patent Application Publication No. 2010-59033 describes using a glass base material manufactured from a porous glass base material and performing elongation such that the tapered portion including an unsintered non-transparent portion is at the bottom and the tapered portion that has already been completely transparently vitrified is at the top, thereby restricting diameter fluctuation near the final portion to be elongated.
With conventional elongation, as shown in FIG. 1, the arrangement of the glass base material is the same as the hanging direction during the process to transparently vitrify the porous glass base material, i.e. the glass base material is arranged such that the tapered portion including the non-transparent portion is at the top and the tapered portion that has already been completely transparently vitrified is at the bottom. In this case, the visible light and infrared light generated in the heated region of the glass base material and in the heating furnace propagate upward through the glass base material and are converted to heat in the non-transparent portion at the top, thereby causing the tapered portion at the top to have a higher temperature than the trunk portion. As a result, the tapered portion at the top is unintentionally stretched near the end of the elongation, which results in the feeding speed of the glass base material being substantially faster than the set speed and causes a greater outer diameter for the glass rod. In the method described in Japanese Patent Application Publication No. 2010-59033, the glass base material is hung with the opposite orientation during the transparent vitrification process and the entire tapered portion at the top is transparent glass, and therefore the light reaching the tapered portion at the top is efficiently released to the outside. In this way, localized temperature increase in the top tapered portion is prevented and the increase in the glass rod diameter near the final elongated portion is restricted.
However, there are two obvious problems with performing the elongation described in Japanese Patent Application Publication No. 2010-59033. The first problem is that after the transparent vitrification of the porous glass base material, when the resulting glass rod is hung upside down, if the tapered portion at the end hanging down is bent, the trunk portion of the hanging glass base material is not oriented straight downward. The bending of the tapered portion occurs when vitrification is performed without the core of the porous glass base material being at the center of the heating furnace during the transparent vitrification process of the porous glass base material.
Japanese Patent Application Publication No. 2001-158626 describes a solution that prevents the cause of the bending at the tip during transparent vitrification, and therefore also prevents the bending. In this way, when the core of the porous glass base material is not at the center of the heating furnace, temperature deviation occurs in the circumferential direction of the porous glass, causing a deviation in the contractive force in the circumferential direction of the porous glass that ultimately results in the bending and eccentricity.
In order to prevent this bending, the core position of the porous glass base material is adjusted to match the center of the furnace. In this way, the deviation in the temperature and contractive force in the circumferential direction of the porous glass base material is eliminated, and therefore the eccentricity and bending can be eliminated. With this method, the bending of the trunk of the base material can be sufficiently decreased, but it is difficult to completely eliminate the bending near the tip. This is because, when the region near the tip at the bottom is heated, there is no weight further below the portion being heated, and therefore deviation in the contractive force of the porous glass caused by even a small deviation in the temperature in the circumferential direction directly affects the bending. When heating the trunk portion, the weight of the tapered portion further below that has already been vitrified pulls downward, and therefore even if there is a small deviation in the contractive force, the force of the weight further blow acts to straighten the bending, and so it is more difficult for significant bending to occur when compared to the tip of the tapered portion.
If there is a bend on the lower-hanging side, this has a large impact on the elongation process. For example, if there is a bend with an angle of 0.2 degrees in the tapered portion of a glass base material with a length of 3000 mm and the side having this bend is oriented at the top when hung, the bottom end of the glass base material can be theoretically skewed from the center line of the hanging portion by up to 10 mm. When the technique for reducing bending described in Japanese Patent Application Publication No. 2001-158626 is not used, the variation in bending amount increases and can reach up to 3 degrees or more. Depending on the flexibility of the hanging portion, the actual skew amount from the center line of the hanging portion can be less than the theoretical value, but when there is such skew, the glass base material contacts the heating furnace and large bending stress is placed on the hanging portion, thereby increasing the risk of damage to the hanging shaft. Furthermore, since the glass base material is not hanging completely vertically, the bending in the glass rod is increased.
The method described in Japanese Patent Application Publication No. 2010-59033 includes, during the transparent vitrification process, causing a portion of the upper tip to remain as a non-transparent glass portion, and heating this non-transparent glass portion first by inverting the orientation when hanging the glass base material during elongation. The second problem is that, at the start of elongation, when heating the non-transparent glass portion, the non-transparent glass portion could crack or break. Pores in the porous portion of the non-transparent glass portion where the sintering has not significantly progressed are filled with air. As the vitrification proceeds from the surface while heating this portion, air becomes trapped inside and unable to escape, and then when the temperature is further increased, the trapped air expands and causes swelling or cracking in the non-transparent glass portion.
The present invention aims to provide an elongation method for a glass base material having a large diameter, and in particular a glass base material that has a transparent glass tapered portion at one end of the trunk portion, a glass tapered portion including a non-transparent glass portion at the other end of the trunk portion, and high outer-diameter precision.